The present invention relates to equipment for optically coupling a semiconductor and an optical fiber for optical fiber communication.
Optical coupling equipment for coupling radiation light from a semiconductor laser diode to an optical fiber or coupling radiation light from an optical fiber to a photodetector is indispensable in constructing an optical fiber communication system. Particularly, to couple radiation light from a semiconductor laser diode efficiently to a single mode fiber, the prerequisite is that the laser diode and fiber be positioned with extremely high accuracy. On the other hand, to couple a semiconductor laser diode and a single mode fiber, equipment which is simple and miniature and feasible for quantity production is needed.
Optical coupling from a semiconductor laser diode to an optical fiber may be implemented by, e.g., equipment using an optical fiber having a hemispherical end, as proposed by Kuwahara in APPLIED OPTICS, Vol. 19, No. 15, 1980, pp. 2578-2583. The hemispherical end or tip is configured by chemical etching or similar technology and on which light is incident. The optical fiber with such a tip is located in the vicinity of the radiation end of a semiconductor laser diode, so that radiation light from the laser diode may be coupled to the fiber. The hemispherical end of the fiber condenses the radiation light to enhance coupling efficiency. In addition, such coupling equipment is miniature and simple since it does not need a lens.
However, the problem with the coupling equipment using a fiber having a hemispherical tip is that the laser diode and fiber have to be positioned with high accuracy, compared with equipment using a lens. For example, to couple a semiconductor laser diode and a single mode fiber, the positional deviation of the fiber and the radiating portion of the laser diode relative to each other should be about less than 0.5 .mu.m in the direction perpendicular to the optical axis and less than several microns in the-direction parallel to the optical axis. Generally, the tolerance of the outside diameter of a single mode fiber and the concentricity of the core each has irregularity of about 1 .mu.m. Also, a mount for mounting the laser diode and fiber involves errors. Therefore, even when the laser diode and fiber are mechanical positioned on the mount by use of marks or similar references, they are displaced relative to each other and, therefore, fail to set up optimum optical coupling. It follows that optical coupling has to be set up by adjusting the position while monitoring the radiation light from the laser diode. This kind of scheme, however, requires the laser diode to oscillate and results in time-consuming adjustment. Moreover, even if the laser diode and fiber are brought to and fixed at optimum positions, they are displaced due to varying ambient temperature and aging, degrading coupling efficiency.
Further, an optical loss ascribable to Fresnel reflection occurs on the hemispherical tip of the fiber. Since the hemispherical tip is located in close proximity to the laser diode, the Fresnel reflection light from the hemispherical tip is incident roan active layer included in the laser diode. As a result, the oscillation of the laser diode becomes unstable to obstruct stable communication.
The influence of the Fresnel reflection light on the end of the fiber is a problem not only on the transmission side but also on the reception side. Specifically, when the radiation from the fiber is coupled to the photodetector, the Fresnel reflection light on the end of the fiber or that of the photodetector sometimes results in echoes due to multireflection on the transmission path.
Besides, conventional equipment for coupling a semiconductor laser diode and an optical fiber needs an automatic power control (APC) circuit for maintaining the light output from the fiber constant. Specifically, the light output power of a semiconductor laser diode falls when the ambient temperature is high. The APC circuit controls an injection current by monitoring the radiation light from the rear end of the laser diode, such that the output light remains constant.